1. Dipartimento di Chimica, University of Perugia, via Elce di Sotto 8, 06100 Perugia, Italy, Institut de Chimie Minérale et Analytique, University of Lausanne, BCH, CH-1015 Lausanne, Switzerland, and Department of Chemistry, Faculty of Science, Kyushu University, Hakozaki, Higashi-ku, Fukuoka 812, Japan